1. Field of the Invention
The present invention relates to a method for forming a device substrate and more particularly to a method for forming a device substrate with a metal back-gate and the structure formed thereby.
2. Description of the Related Art
Laminating a thin metal layer (e.g., tungsten (W)) between silicon-based materials such as a gate oxide and a buried oxide is a key step to making substrates for double-gate devices with a metal back-gate. Because of its flexibility with all kinds of materials, even if the materials are polycrystalline, amorphous, or single crystalline but with very different lattice mismatch, wafer bonding is a promising approach to make this multilayer structure.
However, for the bonding process, after the room-temperature joining step, a thermal treatment at 1100xc2x0 C. is commonly used to enhance the bonding strength. Due to the weak adhesion at the interface between metals and silicon oxide as a result of their chemical and physical incompatibility such as thermal mismatch, the stacked layers are very likely to disintegrate during the high temperature bonding anneal in the form of delamination at weak interfaces. This problem has been observed experimentally and this conventional process is schematically shown in FIG. 1. In the case of tungsten bonding with silicon dioxide, tungsten reacts with oxygen from silicon dioxide to form tungsten oxide (WO3), which its volume increases dramatically (3xc3x97) and causes delamination, during high temperature bonding anneal.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a method (and structure formed thereby) in which a device substrate is formed with a metal back-gate.
In a first aspect of the present invention, a method for forming a device substrate having a metal back-gate, includes providing a substrate, forming a metal back-gate on the substrate and forming an intermediate xe2x80x9cgluingxe2x80x9d layer on the back-gate to enhance adhesion between the back-gate and the subsequent layers (e.g., especially two layers with very different chemical and physical properties).
With the unique and unobvious aspects of the present invention, making substrates for double-gate devices with a metal back-gate can be performed including using wafer bonding and despite after the room-temperature joining step, a thermal treatment at 11001xc2x0 C. is used to enhance the bonding strength. That is, even with chemical and physical incompatibility of layers, the stacked layers are not likely to disintegrate during the high temperature bonding anneal, and delamination will not occur at the interfaces.
Thus, the present invention resolves the above-mentioned and other problems of delamination between W and low temperature oxide (LTO) during bonding anneal by improving the adhesion between these two incompatible materials with several innovative processes.